Composite materials are typically used in structures, e.g., aircraft, trains, vehicles, industrial machines, etc. because of their light weight and strength. Composite panels are examples of such materials and can be of two sheets of one or two types of materials sandwiched about another type of core material. For example, one type of composite panel has two sheets of a material such as graphite-epoxy, para-aramid synthetic fiber epoxy (Kevlar), fiberglass or aluminum, or a combination thereof, sandwiched about a honeycomb core made from materials such as meta-aramid fiber (NOMEX), aluminum, or paper. The resulting composite structure is light, and stiffer than any of its component parts. However, in such composite panels, sound can be radiated very efficiently because the transverse and/or shear wave speed through the structure can be greater than the speed of sound in air. Thus, the composite structure most often has a supersonic transverse and/or shear wave propagating in it. As compared to metallic structures, such composite structures are very efficient in radiating noise. If the composite structure is intended to form or define an interior space, unacceptable noise can be radiated by the composite structure into the interior space. Current methods of addressing this noise problem include the addition of damping material or noise control material into the composite panel to provide the composite panel with sound and vibrational energy absorption means. Examples of noise reduction and damping materials include a limp mass or a visco-elastic layer applied to one or both of the composite panel's face sheets and/or the inclusion of foam within the composite panel's core (e.g., a honeycomb core). However, these extra noise-control materials add cost, weight, and complexity to the composite panel. For example, the combined process of adding damping and fiberglass insulation blankets is labor intensive and adds considerable weight. Moreover, damping tiles are expensive and must be individually installed (by hand) on each panel. Consequently, interior noise control of launch vehicles, aircraft, rotorcraft, and other vehicles poses significant technical challenges as it must take into account weight, cost, and system performance. Thus, at least one technical problem left unresolved is changing the acoustic properties of a composite structure for improved performance with minimum weight, cost, and complexity impact.
Also, conventional composite structures have poor thermal and electrical conductivities requiring extensive additional work to improve these properties. As conventional noise control treatment (for example, in aerospace vehicles) systems typically consists of several non-conductive elements, e.g., damping, fiberglass blanket, acoustic foam, trim panel, isolators, etc. Therefore, extensive additional measures are taken to provide for electrical conductivity to the structure, e.g., into the fuselage, for various purposes, such as incorporating metallic wire mesh for electrical conductivity and grounding in the above solutions. Thus, another technical problem left unresolved is providing simpler, more robust, and improved composite structures with minimum weight, cost, and complexity impact and sufficient electrical and thermal properties that would eliminate or reduce the need for changes in electrical and thermal conductivities.